In the past, electronic devices, such as televisions, stereo equipment, and desktop computers, were powered exclusively by plugging them into an electrical outlet contained in a house or office building. This umbilical attachment to a building meant that these devices didn't move around too much, since they were useless when they weren't plugged in. When a business person traveled, they were forced to leave their prized electronic devices behind.
But travelers wanted to take these devices along with them when they traveled. Manufacturers of these devices saw this need and gradually transformed these immobile devices into battery powered, portable electronic devices. Now a business person can take a battery powered television, stereo, computer, and/or other electronic device with them on a business trip--all in their briefcase.
This advancement in portability has been a boon to travelers, but has not been without its problems. One such problem is that the battery life of these portable devices is often quite short--only a few hours or so. Many business travelers have experienced the frustration of working on a portable computer during a long airplane ride, only to have the battery die before the work is done. Since the traveler knows that their battery isn't going to last very long, they have probably brought along a battery charger to charge their battery when it dies. But battery chargers are normally large, bulky items that must be plugged into a power source, such as a standard wall outlet of a hotel room or office building, before they are useful. Once plugged in, they can often be used to run the computer or other electronic device from the power source while the battery gets recharged.
Unfortunately, the battery, and thus the computer, is worthless until the business traveler gets to a hotel, office building, or other place with a power source.
Many travelers get around this problem by bringing along a fully charged "spare" battery that they can swap with the dead one. While this approach allows the traveler to use their computer longer, it forces them to carry around yet one more piece of equipment when they travel. In addition, once the traveler gets to a place with a power source, they often must make the difficult choice whether to use the battery charger to charge the spare battery, or to use it to run the computer to get the work done. This solution is undesirable to very active business travelers, also known as road warriors, who often spend precious little time in places with power sources that the battery charger can plug into. These road warriors often find themselves in the untenable position of lugging around a portable computer, a battery charger, and two batteries--both of which are now dead--all across the country or the world. All of this equipment sits in the road warrior's briefcase, utterly useless for large portions of their trip.